Adaptive Cruise (i.e., speed) Control (ACC) systems operate much like conventional Cruise Control systems, with the added capability of being able to sense in-path vehicles and to slow the ACC equipped vehicle in response. An ACC equipped vehicle thereby allows its operator to automatically control the vehicle speed, as with conventional Cruise Control, without the necessity of having to deactivate and reactivate control whenever slower traffic is encountered.
As is well known in the art, existing ACC methods and systems use a forward looking range sensor such as radar to sense an in-path vehicle (which may also be referred to as a sensed target or primary target). Based on the radar sensor information, such ACC methods and systems then determine the range and relative velocity (or range rate) of the sensed in-path vehicle. Using the range and range rate, the speed of the ACC equipped vehicle is controlled to maintain a selected following interval between the ACC equipped vehicle and the sensed in-path vehicle. The speed of the ACC equipped vehicle is typically controlled by automatic control of the vehicle throttle actuator. In more advanced ACC methods and systems, vehicle speed may also be controlled by automatic control of vehicle brake actuators. Such ACC methods and systems have the ability to apply a moderate degree of braking to the vehicle to achieve further vehicle deceleration (i.e., in addition to vehicle deceleration achieved via throttle control) in response to an in-path vehicle.
One problem associated with existing ACC methods and systems, however, occurs when the ACC equipped vehicle is traversing a curved path, such as on an a high-speed expressway. In such situations, existing ACC methods and systems inhibit acceleration of an ACC equipped vehicle based on a sensed vehicle yaw rate and a single, constant yaw rate threshold. In that regard, vehicle yaw rate increases as a vehicle enters a curve, and decreases as a vehicle exits the curve. In an ACC equipped vehicle, for vehicle yaw rates less than a threshold yaw rate, the vehicle speed is limited to a first maximum allowed speed value, considered the control speed or the set speed in normal ACC operation. For vehicle yaw rates greater than the yaw rate threshold, the vehicle speed is limited to a second maximum allowed speed value less than the first maximum allowed speed value. As a result, while an ACC equipped vehicle is traversing a curved path, existing ACC methods and systems may allow or inhibit vehicle acceleration somewhat abruptly, thereby causing possible discomfort to the vehicle operator.
Thus, in an ACC system, there exists a need for an improved method and system for controlling the vehicle speed while the vehicle is traversing a curved path. Such a method and system would sense a vehicle yaw rate, and determine a yaw acceleration based on the yaw rate. Such a method and system would further determine a maximum allowed speed of the vehicle on the curved path based on the yaw rate and the yaw acceleration, wherein the maximum allowed vehicle speed would be capable of varying continuously, and limit the speed of the vehicle on the curved path to a value no greater than the maximum allowed vehicle speed. Such a method and system would thus limit the speed of the ACC equipped vehicle on the curved path to a maximum allowed vehicle speed determined based on not only the sensed vehicle yaw rate, but on yaw acceleration as well, thereby allowing for gradual deceleration of the ACC equipped vehicle as it enters the curve, and gradual acceleration of the ACC equipped vehicle as it exits the curve. In so doing, the speed of the ACC equipped vehicle in a curve would be controlled in a fashion similar to manual control of vehicle speed in a curve, thereby providing improved vehicle operator comfort as the ACC equipped vehicle traverses the curve.